【作者】 张晓渝；

【导师】 陈亚杰；

【作者基本信息】 苏州大学 ， 凝聚态物理， 2004， 硕士

【摘要】 磁电子学是当今国际凝聚态物理和材料科学界关注的方向之一，它是小型化，快速化，高存储密度，高灵敏磁性器件的基础。自旋电子学的基础研究有着丰富的物理内涵，应用研究又具有明确的应用目标和广阔的市场前景。目前，非挥发性磁存储记忆器件—磁存储器Magnetic Random Access Memory（MRAM）的研究和开发已成为自旋电子学研究的主要应用目标；而在室温和低磁场下，具有足够大的磁电阻（MR）效应成为实现MRAM器件应用的前提。采用高自旋极化率（P）的材料是产生大的隧穿磁电阻（TMR）效应的关键之一，而半金属（half-metal）铁磁材料的自旋极化率P理论上可达100％。近年来，实验上已制备出许多具有高自旋极化率的氧化物材料，如Fe₃O₄（P=-80％）、CrO₂（P=98.4％）和LaCaMnO₃（P=78％）等，与理论预言的半金属材料十分接近。尖晶石型亚铁磁材料Fe₃O₄不仅具有强磁性，而且具有高达858K的居里温度，在磁带工业、生物医学等领域已得到广泛的应用。目前，作为半金属材料之一，通过纳米点接触实验，测得Fe₃O₄室温，低场（60 Oe）磁电阻为-84％。同时，在与γ Fe₂O₃、Ag、Polymer等材料的复合体中磁电阻效应都得到明显的增强。本文采用Fe₃O₄-C导电颗粒复合的方法，形成新的微结构来调整颗粒界面状态和颗粒间的接触状态，以达到降低外磁场、增强磁电阻效应的目的。 另一方面，随着材料科学的发展，复合材料以其独有的特性受到国内外材料研究人员的广泛关注。复合材料不仅保持了原有各组分的物理性质，而且由于各组分之间耦合等相互作用，显示出比单一组分更加优越的性能。因而，分析复合体微观结构所导致的复合体磁性能变化有着十分重要的意义。 本论文的目的是研究两类复合材料：立方形Fe₃O₄和纳米球形导电石墨颗粒复合体；MnZn铁氧体和SiO₂非磁颗粒复合体。系统地探讨组分对微结构和磁电阻效应的影响，以及复合材料中的磁输运机制；分析复合材料组分对微结构和磁性能的影响。 主要研究内容包括： 1．Fe₃O₄-C颗粒复合物磁性气化物颗粒复合材料的逾渗效应和磁输运性质研究摘要 由立方形的亚磁性Fe3O;颗粒和粒径比Fe3O;小得多的、球形且导电的非磁石墨颗粒组成的复合材料。我们重点研究了这两类不同大小、自旋极化和导电性能相差较大的两相复合材料的磁输运性质。（1）磁电阻与逾渗效应。由于室温下纯Fe3O;粉末颗粒与纯石墨颗粒的电导存在六个量级的差异，因此随石墨含量的增加复合体电导呈现从低到高的转变。实验结果表明，复合体渗流闽值Pc二0.212。并且在逾渗闭值附近，复合体的磁电阻MR出现峰值（MR=一32o/o，T一77K）。本实验主要研究相变附近体系的微观结构对磁电阻效应的影响，探讨导致复合体磁电阻在逾渗闭值附近增强的输运机制。（2）研究了不同温度下Fe3O4一C颗粒复合体的逾渗阂值及其临界行为，从实验上验证了逾渗闭值不仅与颗粒尺寸相关，而且与复合体两组分电导差异的大小相关。同时对实验中出现的非普适临界指数作出了解释，分析了不同温度下的临界指数产生变化的原因。2.Mnzn铁氧体一5 10，磁性颗粒复合体 研究了磁性颗粒复合体磁导率、矫顽力、比磁化强度随磁性颗粒含量变化的 关系。实验表明，随磁性颗粒含量的变化，磁导率和比磁化强度不是作线性变化， 而是在一定区域内存在突变的行为。同时，在这个区域矫顽力出现极大值。分析 认为，磁导率的突变与比磁化强度的突变有着密切的关系，而在突变的区域两侧， 磁性颗粒间磁关联状况的不同，直接导致不同矫顽力机制。因而，在磁导率发生 突变的区域出现矫顽力的异常。更多还原

【Abstract】 Magneto-electronics lies in the area of current scientific interest in condensed matter physics and material sciences, due to abundant physical conceptions and applied potentials. It is a base of magnetic devices with high density and sensitivity. Recently, non-volatile magnetic computer memory (MRAM) is expected to have a large economic impact, and has been significantly focused on its experimenatal research. The MRAM needs to be of a large magnetoresistance (MR) effect at room temperature and lower magnetic field, in order to demands of practical application. However, the large tunneling MR effect is strongly dependent upon electrode material, which is of high spin polarization. On the other hand, half-metallic material is theoretically predicted to be perfect spin polarization, nearly 100%. Indeed, in recent years, some half-metallic oxides have been fabricated and measured, such as Fe3O4 (P=-80%), CrO2 (P=98.4%), LaCaMnO3 (P=78%), etc.. The spinel structure ferrimagnetism FeaO4 provides with not only strong magnetism, but also 858K Curie temperature. Material of FeaO4 has been applied widely in magnetic type, biologic medicine, and other fields. As one of half-metallic materials, in nanometer point contacts experiment, the MR FeaO4 of is -84% at 60 Oe and at room temperature. At the same time, the enhanced MR effect can be observed in FeaO4Ag, FesO4 Y Fe2O3, and Fe3O4/polymer etc. composites. Therefore, it is an efficient way to add the nanometer graphite particles to Fe3O4 granules for enhancing extrinsic MR, due to the formation of new microstructure, which adjusts surface of particle and interface of particles.In the other hand, with the development of material science, many researchers pay much attention to studies on composites, which exhibit not only the properties of single component but also some astonishing properties in composite. Therefore, it is very important to study magnetic properties of composite with the variation of microstructure.The thesis was originally intended to study two kinds of novel composites: (1) cubic Fe3O4 / spherical conducting graphite composite; (2) MnZn ferrite / SiO2 non-magnetism composite. The emphasis is placed on the magnetotransport of the composites that isconcerned with the relation between microstructure and MR effect, and discussion of magnetotransport mechanism in composites. Particular interest is to study influence of component on microstructure and magnetic properties.The main work involves:1. Fe3O4 / C granular compositeThe composite consists of cubic, ferrimagnetism Fe3O4 particles and spherical, conducting, diamagnetic graphite particles. Work is focused on the magnetotransport properties in terms of microscopic mechanisms, in the binary composite with different size, electrical, and spin polarization. (1) Magnetoresistance and percolation effect:There are 6 orders of magnitude conductance difference between Fe3O4 powder and graphite powder at room temperature. Therefore, it may exhibit a transition from high conductance to low conductance with the increasing of FeaO4 particles. The experiment indicates that the percolation threshold is pc=0.2l2 and a maximum MR value reaches -32% at T=77K when approaches increasingly . We mainly study the influence of microstructure on magnetoresistance near percolation region and discuss the mechanisms which lead to enhanced magnetoresistance. (2) Percolation threshold and critical exponentPercolation threshold and critical behavior of FesCVC composite have been studied at different temperature. The experiment indicates that percolation threshold is dependent not only size of particle but also the different of conductance of two components. At the same time, we explain the non-universal critical exponent in composites and variation of critical exponent at different temperature.2. MnZn ferrite /SiCh magnetic composite:Permeability , coercivity and the specific magnetization with the variation of magnetic particles content have been studied. The experiment indicates thatStudy更多还原